The present invention relates to nanometer and other inter-atomic position measurements and the like with the aid of probing sensors relatively moving with respect to periodically undulating atomic or other surfaces, such as gratings and the like, over which position locations are to be determined in real time.
Limitations in prior art laser beam interferometry measurements and similar prior optical probing techniques led to the development of oscillating sensing probes between which and such surfaces, sensing fields were established, relying upon the generation of sinusoidal output voltages measured after passing through the surface from the oscillation-controlled probe that, by comparison of the phase and amplitude of the oscillation controlling and resulting sinusoidal output voltages, enabled the development on a continual basis of positional signals indicative of the position of the probe along the surface, as described in my earlier U.S. Pat. Nos. 5,589,686 and 5,744,799.
As detailed in my said prior patents, real-time continual nanometer scale position measurement data of the location of a sensing probe relatively moving with respect to an undulating surface (an atomic surface or a grating or the like) is achieved through rapid oscillating of the probe under the control of sinusoidal voltages as a sensing field is established between the surface and the probe, producing output sinusoidal voltages by the current generated in the sensing field. As therein detailed, comparison of the phase and amplitude of such output voltages provides positional signals indicative of the direction and distance off the apex of the nearest atom or undulation of the surface. Circuits for developing such positional signals are disclosed in said patents and, where desired feedback is shown effected of the positional signals to control the relative movement of the probe and surface.
There are circumstances, however, where it is desirable to use probing by energy beams, such as by laser beams (and, as later discussed, other energy beams such as electron and ion beams and the like) as distinguished from physical, capacitator or magnetic probes illustrated in said patents; and, indeed, to use the beam energy not only in probing over the surface but as a contributor to the setting up of the sensing energy field with the atomic or other insulating surface itself.
Laser interferometers, indeed, are almost exclusively used presently for high precision position measurements. Practical limits and precision in resolution are, however, being reached for reasons, including those which have been explained in said patents.
A laser scale position encoder which uses a laser source and a holographic grating as a reference scale, for example, utilizes the interference produced between the two first order (+/xe2x88x92) diffracted laser beams generated through a holographic scale. While this method somewhat circumvents the problem of ordinary laser interferometry by reducing environmental sensitivity to shorter laser path length, diffraction theory limits the minimum period of the reference grating that can be used; roughly on the order of the light source wavelength, e.g. 632 nm for Hexe2x80x94Ne lasers. This means that an atomic level of precision is difficult to achieve with this type of laser scale position encoder. Any changes of the beam path length caused by, for example, table tilting and laser wavelength shift, furthermore, will result in rather significant incorrect position measurement. Gap changes between the head and the reference grating will usually either cause significant error of position measurement or no effect on the system due to symmetric optics location. Thus, such cannot be used for multi-axis position measurement systems of the type described in my said patents. Finally, this technique relies on the average effect (usually several mm) of thousands of grating lines and spaces for a resultant interference pattern. While this generally provides better signal-to-noise ratio in measurement, the basic assumptions, like homogeneity of the temperature distribution and periodicity of the grating lines and spaces over such broad area, do not accurately hold, so as to achieve atomic precision (repeatability) in position measurement.
In general, diffraction limits the smallest focusable laser spot size, which is roughly on the order of the laser source wavelength. Recent progress in near field optical technology and other super resolution enhancement technology, however, [J. Tominaga, T. Nakao, and N. Atoda, xe2x80x9cAn approach for recording and readout beyond the diffraction limit with an Sb thin filmxe2x80x9d, Applied Physics Letters, Volume 73, Number 15, October, 1998] has enabled the focusing of the laser beam to a size even smaller than the theoretical limit imposed by diffraction theory.
The present invention in one of its applications, however, takes advantage of such a narrowly focused laser beam, which has now been found also to be utilizable as a beam probe for position measurement, now realizing nanometer precision position measurement through adoption therewith of the different techniques of my said patents.
In other applications, such as optical disc technology and the like, several technologies have been developed to detect an off-track signal, which indicates the distance between the nearest track center and the beam spot center. These include three beam, push-pull and differential phase detection (DPD) methods which are popular for those types of systems. If this type of operation were to be applied to the field of the present invention, however, replacing the optical disc with a holographic grating and providing a displacement sensor, neither the off-track information nor the total beam reflection signal alone can provide information about the nanometer-scale, real-time position measurement results. Such an approach, therefore, cannot be directly applied for position measurement purposes. The present invention, on the other hand, reliably provides both motion direction and position, while using focused laser beam(s).
The current invention achieves its improvement in nanometer position measurement and control by replacing what might be termed the xe2x80x9cfront endxe2x80x9d of the systems taught in my said patents with novel laser beam probing systems which are hereinafter described in detail as the novel beam probing xe2x80x9cfront endsxe2x80x9d substituted for the physical probes of my said patents, with the understanding that the same output voltage comparison and processing circuits used in the systems of said patents are to be understood as employed therewith; such that the details thereof are not repeated herein in order to avoid complicating the disclosure and detracting from the features of novelty of the present invention.
An object of the invention, therefore, is to provide a new and improved nanometer measurement method and system that, through using laser (or other radiation) beam probing, shall not be subject to the above-described and other limitations of prior laser interferometry systems and the like, or of prior physical probes, but that, to the contrary, provide novel beam probing techniques that enable nanometer precision measurement both in motion direction and position.
A further object is to provide such a new system in which the beam probing may be effected both with beam oscillation or scanning, and also, where desired with non-oscillating beams, as well.
An additional object is to provide, also, improved position sensing response and operation particularly advantageous in such laser beam probing systems.
Other and further objects will be explained hereinafter find are more particularly described in connection with the appended claims.
In summary, however, from one of its important aspects, the invention embraces a method of real-time nanometer scale position measurement of the location of a radiation beam probe relatively moving over a periodically undulating surface, that comprises, focusing and impinging the beam upon the surface during such relative movement and receiving the successive reflections of the beam from the surface; generating substantially sinusoidal voltages through the reception of the beam reflections having the periodicity of the surface undulations; comparing the phase and amplitude of the generated sinusoidal voltages with a reference sinusoidal voltage at a related frequency through multiplying the generated and reference voltages to develop positional signals, on a continual basis, indicative of the direction and distance of the beam probe off the apex of the nearest undulation of the surface, and thus the position of the beam probe along the surface.
Preferred and best mode designs and configurations will later be explained in more detail.